ventilation in new shed roof addition

I am adding a "L" shaped addition to exisiting structure ... it runs along east (wall is 14 ft) and north (wall is 12 ft.) sides of house. Roof slope will be 5/12 and catheral ceiling with 2" x 10" joists 24" on center. There will be 2 1/2 inches of air space between insulation and sheathing but not sure how to vent at top end where roof ties into existing structure. And we potentially get lots of snow in winter.

Any ideas would be much appreciated.

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ventilation in new shed roof addition

I'm in the process of a similar project.
In my case the house has a hip roof. I'll be suporting the new shed roof on a ledger board lagged to the wall studs.
I'll be cutting the rafter lookouts off at the birdsmouth.
My shed roof rafters will be 2X8's and I'll be using R20 (6") insulation. This will allow 1 1/2" of space for airflow. The air will flow over the existing wall plate and into the attic. And then of course, out through the roof vents.
If you are building to a gable, then this is not possible.
Is necessary for you to maintain the ceiling to the rafters?
If at some point, you can have the ceiling level off to form a small attic, you could then place gable vents in the sides.
Failing that, perhaps you could lay 2X4 stringers on and perpendicular to the rafters. then nail the roof sheeting to these. This would allow lateral air movement to the side soffits!

ventilation in new shed roof addition

Wildie

My addition is also going to have a hip roof as well (at the corner) and gable ends so the stringer idea makes sense. Although it is not necessary for the ceiling to attach to rafters, it's small space and I think it will look/feel better. I like the idea of stringers ... that seems like a good possibility.

What do you think about this idea from a friend who has done lots of building ... leave a 3/4 inch gap at the top (don't bring sheathing all the way to the wall) and having this all along the top edge will create the necessary channel for air flow and then vents in gable ends. The flashing will cover and protect from the elements.

And you say you are leaving 1-1/2 inches air space ... maybe I don't need as much as I was thinking (2-1/2 inches). There will be about a two foot overhang and space for air to flow over the top plate up over the insulation and out the 3/4 inch space.

ventilation in new shed roof addition

Quote:

Originally Posted by berta

Wildie

My addition is also going to have a hip roof as well (at the corner) and gable ends so the stringer idea makes sense. Although it is not necessary for the ceiling to attach to rafters, it's small space and I think it will look/feel better. I like the idea of stringers ... that seems like a good possibility.

What do you think about this idea from a friend who has done lots of building ... leave a 3/4 inch gap at the top (don't bring sheathing all the way to the wall) and having this all along the top edge will create the necessary channel for air flow and then vents in gable ends. The flashing will cover and protect from the elements.

And you say you are leaving 1-1/2 inches air space ... maybe I don't need as much as I was thinking (2-1/2 inches). There will be about a two foot overhang and space for air to flow over the top plate up over the insulation and out the 3/4 inch space.

Thanks for your comments ...

Unless I misunderstand, its the channel formed would have a cross section of 1/2"X3/4" to funnel the air from the rafter bays (14 1/2"X1 1/2" times the number of bays)! Seems like a tight squeeze to me.
I would think that the vent cross sectional area should be equal to the cross section of the total rafter bays.
I'm not a framing carpenter and perhaps someone with experience in this line, would be able to enlighten us!